In photovoltaic (PV) installations, the power-generating PV generators are connected to one or more remotely mounted inverters via direct current lines which are often very long. In this case, devices such as protective devices or measuring devices which are close to the generator and communicate with the inverter or other control units installed close to the inverter are often provided. In addition to conventional wired communication methods via separate signal lines or serial and/or parallel bus or network connections or radio connections, communication can also take place via the direct current lines which are used to connect the PV generator to the inverter. Such communication for the purpose of transmitting data via lines for energy transmission is also known as powerline communication (PLC). In this case, the communication signal is coupled onto the lines for energy transmission as an HF signal and is coupled out again and analyzed at the receiving end by means of corresponding coupling-out means.
Another use of high-frequency signals in PV installations is described in the documents DE 10 2010 036 514 A1 and WO 2012/000533 A1. A high-frequency signal fed into a photovoltaic installation is coupled out again and evaluated at another location. For example, the signal response which has been coupled out is compared with reference data which were recorded during fault-free normal operation of the photovoltaic installation. Events which impair the operation of the PV generator are inferred on the basis of the evaluation of the signal response. Such events are, for example, theft of one or more PV modules or contact problems which have occurred.
The document DE 199 40 544 A1 discloses a circuit arrangement for coupling an HF signal for transmitting data onto an AC low-voltage network, such as a typical domestic electrical installation. In this case, a network for matching the impedance to a transformer is connected downstream of an output amplifier for the transmission signal, the transformer transmitting the signal to the alternating current lines of the domestic installation. The circuit arrangement has a control device which uses a current measurement inside the transmission network to vary the voltage amplitude of the HF signal at the input of the transmission network via the output amplifier in such a manner that the HF signal on the alternating current lines of the domestic installation has a constant voltage amplitude of a predefined level. Receivers for the HF signal are arranged in a manner substantially parallel to loads inside the domestic installation. The constant voltage amplitude ensures that an equally strong signal is received at any of possibly a plurality of receiving units connected in parallel.
In PV installations, the PV generator usually has one or more series circuits comprising a plurality of PV modules, a so-called string. A plurality of such strings which are connected in parallel with one another often form the PV generator. Receivers for data, which are transmitted on the direct current lines starting from an inverter or a central control device using an HF signal, are present under certain circumstances at the level of individual PV modules. Data transmission in the opposite direction from a transmitter arranged in the string to a receiver positioned close to the inverter is also customary. Transmitting and/or receiving devices are referred to as communication units below. They may be set up both for unidirectional and for bidirectional data transmission.
Owing to the series connection of the PV modules inside a string and the parallel connection of two or more strings, the signal strength which can be coupled out at an individual PV module in known circuit arrangements for coupling in the HF signal is often low and is also not known a priori even if the transmission signal strength is known. This is explained in more detail below by way of example for the case of communication from the inverter in the direction of the PV modules taking into account inductive coupling-in and coupling-out of the HF signals. In the case of inductive coupling-out, the signal strength of the HF signal which has been coupled out results from the current intensity of this signal at the location of the receiver (here: the PV modules). If, during transmission, the HF signal at the location of the inverter is now coupled onto the direct current lines with a constantly predefined voltage amplitude, for example, a corresponding current intensity of the HF signal is established depending on the impedance inside the DC circuit. The signal strength of the HF signal which has been coupled out is therefore dependent on the impedance in this DC circuit and is therefore generally different for different PV installations. If, in contrast, during transmission, the HF signal is coupled onto the direct current lines in such a manner that it has a predefined current amplitude at the coupling-in location, a predefined signal strength of the HF signal which has been coupled out is also ensured at the location of the PV modules in the case of an unbranched DC circuit. However, this is no longer the case if the DC circuit of a PV generator has branches, for example in the form of a plurality of strings connected in parallel. In this case, the current intensity of the HF signal coupled in at the location of the inverter is divided among the individual strings at the branch points depending on the impedances of the strings. This results in a signal strength which can be coupled out at an individual PV module, is generally low and is not known a priori, thus making powerline communication significantly more difficult and even preventing it in the worst-case scenario.
Taking into account the above consideration, the most demanding framework conditions of a PV installation having a plurality of strings connected in parallel should be used as a basis for robustly operating powerline communication. In a corresponding PV installation, the DC circuit is formed by different branches of the PV generator, the direct current lines and the input stage of the inverter. In this case, the line lengths and the line routing, the impedances of the various strings which are dependent, inter alia, on the lighting situation and the number of PV modules connected in series and also, under certain circumstances, the input impedance of the inverter are decisive for the system impedance. In contrast to the AC low-voltage network of the domestic installation in which the system impedance is generally low and is also virtually constant, considerably higher and greatly varying impedances may occur in the DC circuit of the PV installation. These impedances vary not only from PV installation to PV installation but also inside a particular PV installation as a function of the time. The type of PV modules used, in particular whether they are modules based on polycrystalline or monocrystalline semiconductor material or so-called thin-film modules based on amorphous or microcrystalline semiconductor material, also has a great influence on the impedances and their variation.
In order to nevertheless achieve powerline communication with satisfactory reliability in PV installations, the highest possible signal strength has hitherto been coupled onto the direct current lines, on the one hand, and a received signal has been amplified in a complicated manner after being coupled out, on the other hand. However, an excessively high transmission level is no longer compatible with EMC (electromagnetic compatibility) guidelines under certain circumstances. Amplification carried out in each of the receivers is material-intensive and is therefore costly and, at high gain factors, is additionally also susceptible to interference from irradiated interference signals which are likewise amplified.
The document WO 2011/085952 A2 discloses an alternative method for transmitting data via lines for energy transmission, which method achieves transmission which is as unsusceptible to interference as possible. In this case, provision is made of an initialization phase in which a test signal is transmitted, which test signal is evaluated in the receiver with regard to its signal strength, in particular an amplitude. Depending on the evaluation result, a repetition rate is determined which indicates how often the data to be transmitted are repeated inside a data message. In the receiver, the repeatedly received signals are added in the correct phase in order to increase the signal-to-noise ratio, as a result of which it is possible to achieve the robust data transmission which is unsusceptible to interference. However, a complex receiver for adding the data received in succession in the correct phase is required.